1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a device and method for gripping and detaching a top nozzle as an assembled unit from a reconstitutable fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies which receive support and alignment from upper and lower transversely extending core support plates. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by a plurality of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles to thereby form an integral fuel assembly. The guide thimbles extend slightly above and below the ends of the fuel rods. Additionally, fuel assemblies have also employed a hold-down device to prevent the force of upward coolant flow from lifting the fuel assembly into damaging contact with the upper core support plate, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. One such hold-down device is illustrated and described in the first application cross-referenced above.
During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of the integral assembly of the guide thimbles attached to the top and bottom nozzles, it is difficult to detect and remove the failed rods.
In view of the high costs associated with replacing fuel assemblies, considerable interest has arisen in reconstitutable fuel assemblies in order to minimize operating and maintenance expenses. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle. One fuel assembly construction adapted to be reconstitutable is the fuel assembly with the hold-down device as disclosed in the first application cross-reference above. The method of reconstituting this fuel assembly is illustrated and described in the third application cross-referenced above.
The top nozzle subassembly of the fuel assembly disclosed in the first cross-referenced application, which includes the hold-down device as an integral part thereof, is basically made up of a coil spring disposed about the upper end of each guide thimble and sandwiched between a lower adapter plate and an upper hold-down plate. The lower adapter plate is slidably mounted on the guide thimble and its downward movement is limited by a lower retainer attached on the guide thimble. The upper hold-down plate has a passageway matched with each guide thimble with an internal ledge defined in the lower portion of the passageway. The guide thimble is received in the passageway so as to mount the hold-down plate on the thimble for slidable movement therealong. An upper retainer is attached on the upper end of the guide thimble and disposed within the passageway of the hold-down plate. The upper retainer cooperates with the internal ledge of the passageway to limit the upward movement of the hold-down plate along the guide thimble. The construction is such that the upper end of the guide thimble, along with the upper retainer, are permitted to reciprocate within the passageway of the hold-down plate, thus allowing for thermal growth of the guide thimble. The coil springs are held in a state of compression between the lower adapter plate and the upper hold-down plate.
The method for reconstituting the top nozzle subassembly without its hold-down device coming apart, as disclosed in the third cross-referenced application, includes the following operative steps. First, a force is applied to the upper hold-down plate to move it downwardly which compresses the springs. Second, with the force being maintained on the hold-down plate, an in-tube cutter is used to sever the upper end of each guide thimble at an axial location below where the thimble is bulge fitted or connected to the upper retainer. Third, after severing is completed, a pair of container fingers are applied to the top nozzle so as to extend above the upper surface of the hold-down plate and below the lower surface of the adapter plate. Then, the force is slowly removed allowing the springs to expand and push the hold-down plate upwardly into abutting contact with the upper ends of the fingers. Finally, with the container fingers in place, the top nozzle subassembly is lifted, generally vertically, upwardly and off the severed guide thimbles as a unit. The severed upper ends of the thimbles and the upper retainers are carried away in the passageways of the hold-down plate of the removed top nozzle subassembly. By reversing the above steps, the top nozzle subassembly can be remounted as a unit back on the guide thimbles, except that now instead of the severing step, new upper retainers similar to the old ones are inserted into the passageways and attached to the upper ends of the guide thimbles.
Notwithstanding the generally acceptable performance of the above-described top nozzle subassembly and method of removing the same as a unit during reconstitution of the fuel assembly, certain problems point to the need for an improved device for removing the top nozzle. First, the prior method of removing the top nozzle fails to provide a mechanism to overcome the slight interference fit formed between the outside surfaces of the guide thimble upper ends and the inside surfaces of the upper retainers due to the bulging operation for attaching the retainers to the guide thimbles. In other words, some means is necessary to apply a force on the retainers to pull them free of the portions of the guide thimbles immediately below the location at which the thimble upper ends were severed. Second, the prior method fails to provide a single device capable of imposing the required force on the hold-down plate, holding the force so as to keep all of the hold down springs in a compressed state, and then gradually releasing the force. Third, the prior method does not provide a single device useful to both grip the top nozzle subassembly and remove it from the fuel assembly. In summary, a need exists for a single device which would provide all of the above-described functions in removing the top nozzle subassembly from the reconstitutable fuel assembly.